The present Invention relates generally to novel radar processors and, more particularly, to radar Doppler processors which exhibit maximum clutter rejection and low minimum detectable velocity with minimal loss.
Presently available radar Doppler processing techniques to distinguish moving from fixed targets include moving target indicator processors (MTI), weighted fast Fourier transform processors (FFT), and cascaded MTI and FFT processors. The criteria used to evaluate the performance of a particular Doppler processor are the improvement factor (IF) which is a measure of the clutter rejection capability, the signal processing loss which is a measure of the loss in signal to noise ratio from the ideal, and the minimum detectable velocity (MDV) which is a measure of the ability to detect low velocity targets or targets whose velocity vector is nearly tangential to the direction of the radar. The limitations and disadvantages of the above-mentioned processors are primarily in the areas of the signal processing loss and MDV response.
The conventional MTI canceller delays the returns of a given transmitted pulse and subtracts them from the returns of the next transmitted pulse. N cascaded cancellers are equivalent to an (N+1) sample transversal filter with weights corresponding to the Nth-degree binomial coefficients with alternating signs.
In general, the binomial weights for such a filter are: ##EQU1## and N.sub.c is the number of cancellers.
For a triple canceller (N=3), the binomial weights would be (1, -3, 3, -1) which, when normalized by dividing by the absolute value of the largest binominal coefficient, would be (0.333, -1.0, 1.0, -0.333).
FIG. 1 shows the filter gain normalized to the maximum filter gain versus Doppler frequency normalized to the radar pulse repetition frequency (PRF) for 1 to 7 MTI cancellers (N=1 to N=7). By cascading MTI cancellers the clutter-rejection null (centered around 0 and 1 on the horizontal axis) is broadened as shown by FIG. 1. For a given clutter spectral distribution, the broader null yields a better improvement factor as shown by FIG. 2 which is a plot of the improvement factor (IF) as a function of the clutter spectral width normalized to the radar pulse repetition frequency. The penalty paid for the gain in the improvement factor is a poor MDV response as well as wide blind velocity regions which occur when the Doppler frequency is a multiple of the PRF.
Another concurrent problem with the poorer MDV response is the increase of the loss as the number of cancellers cascaded increases as given by: EQU Loss=10 log (N.sub.c +1). (2)
FIG. 3 is a plot of the loss given by Equation (2) as a function of the number of cancellers (N.sub.c). The loss corresponds to the loss of an MTI. However, if the MTI is followed by coherent or noncoherent integrators, this loss can be reduced, but not eliminated.
One method of improving the poor MDV response of the MTI is by shaping the "skirt" of the filter by various weighting schemes. For example, FIG. 4 illustrates the amplitude response for three weighted MTI's: the classical three-pulse canceller (curve 1), the five-pulse delay-line canceller with "optimum" weights (curve 2), and a 15-pulse Chebyshev design (curve 3). In FIG. 4, the factor 1/T is equal to the PRF. This method improves the MDV response, but the shaping requires longer processing times (which implies more pulses processed) which in turn increases the loss.
The weighted FFT gives a better MDV response than the MTI, but it also produces 2-3 dB losses because of the weighting function which lowers the filters' sidelobes everywhere, not just in the vicinity of the clutter. For example, FIGS. 5(a), 5(b), and 5(c) illustrate the amplitude responses as a function of normalized Doppler shift for filters number 33, 25, and 5 of a 64-pulse FFT Doppler processor with-70 dB Chebyshev weights. For each filter the overall loss is -2.2 dB.
Heavy weighting (to control the level of the sidelobes) is needed for the required improvement factor. The result is a wider main lobe with its concurrent poorer MDV response because the filters closer to the main lobe clutter reject less clutter causing the output clutter residue to be dominated by the main lobe clutter. The effect of this domination is a reduced detectability for the low velocity targets in the filters close to the main lobe clutter.
The cascaded MTI/FFT processor reduces the main lobe clutter by cancelling the main lobe clutter with the MTI prior to FFT filters as described above. This results in less signal processing losses since the FFT may need to be weighted less or not weighted at all. However, the MDV response is poorer because of the response of the MTI. Increasing the number of MTI cancellers improves the improvement factor, but at the expense of further deterioration of the MDV response. Also, the improvement is not linear. The more MTI's cascaded, the less improvement gained as shown in FIG. 1 and described above. Again, as described above, weighting the MTI can be used to improve the MDV, but the loss associated with the increased number of pulses that must be processed diminishes the gains that were the original reason for cascading.